Due to their low cost and appealing appearance, composite films are commonly used and referred to in the trade as “packaging films” and/or “labeling films.” For example, in the packaging of certain types of foods, a composite film may be used having three or more layers, wherein one or both of the outer layers serves primarily as a heat seal layer, while the other layers may provide other desirable properties, such as structural integrity, barrier, appearance, slip, sealability, printability, and/or other functions.
Composite films are also used in a wide variety of graphical, print media, and industrial applications. Composite films are commonly prepared using polymeric components, such as polypropylene, polyethylene, and other polymer compositions. Coatings, additives, and/or surface treatments may be included to enhance the performance of the polymer films or to overcome some of the limitations of such films. For example, polymeric films may be difficult to print with water-base inks, so a film surface may be coated with a printable coating, treated to increase the surface energy, or provided with an additive in the skin layer to render the surface more printable.
As polymeric films are commonly formed from thermoplastic polymers, such films have limitations with respect to their tolerance of heat and heat related treatments and applications. Too much heat may cause shrinkage, distortion, and sticking to processing equipment. Similarly, such films tend to be rather poor insulators against heat migration.
Polymeric films also are commonly relatively transparent or clear unless rendered opaque or translucent, such as by fillers, cavitation, metallization, and/or coloring agents, such as whitening agents. One drawback to such components is that they may adversely alter the aesthetic properties of a film, such as to cause the films to exhibit a matte surface, reduced gloss, or reduced light transmissibility. A glossy, white background behind printing or images may desirably contribute to a higher quality print result as compared to a film having reduced whiteness. Increases in the loading of fillers or coloring agents may also contribute to mechanical performance problems, such as increased dusting, abrasion, or filler build-up on the processing equipment or on the film surface. A skin layer may be applied over the filler-containing layer, but one drawback to such skin layers is possible loss of whiteness and light transmission due to such skin layer.
U.S. Pat. No. 5,244,861, entitled “Receiving Element for use in Thermal Dye Transfer,” provides an example of the prior art. The '861 patent discloses a thermal dye transfer receiving element comprising a base having thereon a dye image-receiving layer, the base comprising a composite film laminated to a support, the dye image-receiving layer being on the composite film side of the base. The composite film comprises a cavitated, thermoplastic core layer, and at least one substantially void-free thermoplastic surface layer.
It is desired to produce a polymeric film that is useful for thermal dye transfer printing, having improved compressibility, insulating properties, gloss, and whiteness, without the adverse effects of merely increasing the loading of whitener in a film layer. The film should also facilitate improved printing and processing speed and insulating properties. Improved image quality and processing speed may facilitate lower cost and allow dye diffusion thermal transfer printing to compete with other printing technologies, such as ink jet.